Where to find postcards in Baltimore?

I am taking part in a project called PostCrossing and I need to find some postcards to send. I have easy access to Towson and any areas south of Towson, including some of Baltimore City and the Inner Harbor. I also have easy access to many national and regional chains such as Giant, Walgreens, and Rite-Aid. My friend from California said that she found postcards at CVS and Rite-Aid. I have been to these stores a few times but I have never seen postcards (however, it is very possible that I am just not looking in the right areas of the store). Are there any stores that you know of in the Baltimore area that sells postcards? I would prefer for them to be Baltimore, Maryland, or USA themed, but honestly any postcards are fine at this point. Thank you!Where to find postcards in Baltimore?
Plenty to choose from at the National Aquarium. You can get some at the Camden Yards store too. You do not have to enter the stadium to get in.